The high lumen per watt efficacy and long life inherent in fluorescent lamps has given rise to the development of a number of different types of small compact fluorescent lamps in the lighting industry. These small, efficient, and long life lamps are gradually making inroads into lighting applications that have traditionally been dominated by incandescent lamps. Wattage per unit surface area is higher than for conventional fluorescent lamps so that compact fluorescent lamps operate hotter.
Examples of compact fluorescent lamps are disclosed in U.S. Pat. No. 4,374,340, which issued to Bouwknegt et al on Feb. 15, 1983; U.S. Pat. No. 4,426,602, which issued to Mollet et al on Jan. 17, 1984; and U.S. Pat. No. 4,481,442, which issued to Albrecht et al on Nov. 6, 1984.
A number of lighting fixture manufacturers have designed reflector-type adaptors that utilize compact fluorescent lamps as alternatives to R or
type reflector lamps. Such lamps in spot or flood varieties are often used for localized display lighting in stores, or as ceiling-mounted recessed down lights in buildings. The use of inherently hotter compact fluorescent lamps in such enclosed or convection-restricted fixtures results in operating temperatures that were seldom, if ever, previously encountered by fluorescent lamps. By way of example only, glass seal press temperatures over 160 degrees Celsius have been measured on some compact fluorescent lamps in such fixtures.
It is conventional to form the lead-in wires of the lamps from a nickel-iron alloy comprising, for example, 50.5% by weight nickel and having an iron oxide layer on the surface of the wire to improve the hermetic seal. U.S. Pat. No. 4,204,137, which issued to Roy on May 20, 1980, teaches the formation of an oxide film on a nickel-iron portion of the lead-in wire passing through the pinch region of the flare. Normally, a very good seal match is obtained between soda-lime glass and pre-oxidized nickel-iron alloy lead-in wires.
However, it has been discovered that when higher wattage compact fluorescent lamps having pre-oxidized nickel-iron lead-in wires are operated in these hotter fixtures, seal leaks may develop. Although not completely understood, it is believed that the combined effect of the ultraviolet radiation produced in the lamp and increased seal temperatures on the oxide layer causes the stress conditions of the seal to change during lamp operation.